1. Field of the Invention
The present invention relates to a structure of a semiconductor laser device used for optical disk apparatus such as a CD-R/RW drive, a DVD-RAM drive and an MD drive, and the like.
2. Description of Related Arts
FIG. 5 is an end face view of a ridge type semiconductor laser device formed of ALGaAs group materials.
On a substrate 1, a lower cladding layer 2 is formed. An active layer 3 is formed on the lower cladding layer 2. An upper first cladding layer 4 is formed thereon. An etching stop layer 5 is formed thereon. Formed on the etching stop layer 5 is a ridge-shaped upper second cladding layer 7, and on both sides thereof, a light confining layer 6 is formed. Further, a contact layer 8 is formed on the upper second cladding layer 7 and the light confining layer 6.
FIG. 4 is a schematic side view showing a shape of a conventional semiconductor laser device. The members shown in FIG. 4 corresponding to those shown in FIG. 5 are assigned the same reference characters with those of FIG. 5. However, in FIG. 4, the illustration of the structure below the upper second cladding layer 7 and the light confining layer 6 is omitted.
The contact layer 8 is not formed on the whole surface of the upper second cladding layer 7 and the light confining layer 6. That is, no contact layer 8 is formed in a certain range in each of regions in the vicinity of a laser emitting end face A and in the vicinity of a reflective end face B, thereby providing current non-injection regions CL, CR which are not excited by current injection. Current non-injection lengths WL, WR, which are the lengths of the current non-injection regions CL, CR along the upper second cladding layer 7 respectively, are substantially equal to each other. When the resonator length L is 500 xcexcm, both of the current non-injection lengths WL, WR are set about 40 xcexcm.
FIG. 8 is a schematic sectional view for explaining processes of cutting or singulating a plurality of semiconductor devices each having the abovementioned structure out of a larger semiconductor substrate. The semiconductor laser substrate here means a structure in which layers from a lower cladding layer 2 to a light confining layer 6 and an upper second cladding layer 7 are formed on a substrate 1 and contact layers 8 are formed at predetermined positions thereon. In such a semiconductor laser substrate, a plurality of piece regions D4 (one piece region D4 is shown with oblique lines in FIG. 8) each corresponding to a single semiconductor laser device shown in FIG. 4 are laterally and longitudinally connected with one another in a grid-like arrangement.
By cutting such a semiconductor laser substrate at cutting positions C, a plurality of pieces of semiconductor laser devices each having a sectional shape shown in FIG. 4 can be obtained. The cutting position C is set substantially at the center of the region having no contact layer 8. Consequently, each of cut-out pieces has a side shape in which the current non-injection length WL on the laser emitting end face A side and the current non-injection length WR on the reflective end face B side are substantially equal to each other as shown in FIG. 4.
The current non-injection regions CL, CR are provided in order to prevent heat generation due to occurrence of non-radiative recombination in the end portions of the semiconductor laser device at the time of laser emission. That is, if the contact layer 8 is elongated to both of the end faces A, B and current is injected up to these portions, non-radiative recombination occurs to cause heat generation especially at the laser emitting end face A. Due to the heat generation, the band gap becomes small, and thereby laser light absorption increases, which causes further temperature rise. Repetition of this brings so-called COD (Catastrophic Optical Damage), namely, melting of the end face or faces of the semiconductor laser device, which results in destruction of the semiconductor laser device.
In a semiconductor laser device having a limited resonator length L, by making long the total current non-injection lengths WL+WR, the effect of non-injection increases in correspondence therewith and the COD level becomes high. However, if the total current non-injection lengths WL+WR are excessively long, the effective resonator length Le (see FIG. 4), which is the length of the current injection region, becomes short. Accordingly, the current density becomes large and heat generation in the semiconductor laser device increases, so that the COD level is lowered. As a result, disadvantageously, no high output power laser light can be obtained.
Especially in a small-sized semiconductor laser device, since the resonator length L (see FIG. 4), which is the whole length of the device, is short, the total current non-injection lengths WL+WR cannot be made so long for ensuring a certain effective resonator length Le. This results, disadvantageously, in that the COD level becomes low and no high output power laser light can be obtained.
On the other hand, relating to the two end faces A, B of a semiconductor laser device, heat generation at the time of laser light emission occurs mostly on the laser emitting end face A side, and heat generation on the reflective end face B side is not so serious. Therefore, it can be judged that the current non-injection region CR on the reflective end face B side does not contribute so much to the improvement of the COD level.
An object of the present invention is to provide a semiconductor laser device in which, by providing a long current non-injection region on the laser emitting end face side, the COD level can be improved and thereby a high output power laser can be obtained.
A semiconductor laser device according to a first aspect of the present invention comprises a lower cladding layer, an active layer and an upper first cladding layer stacked in this order on a compound semiconductor substrate, a ridge-shaped upper second cladding layer provided on the upper first cladding layer, a light confining layer provided on both sides of the upper second cladding layer, and a contact layer provided on the upper second cladding layer, and the device has two end faces thereof with respect to the longitudinal direction of the ridge-shaped upper second cladding layer, namely, a laser emitting end face and a reflective end face, and a current non-injection region having no contact layer is provided only on the laser emitting end face side of the two end face sides.
According to this invention, by omitting a current non-injection region on the reflective end face side, a long current non-injection length can be ensured on the laser emitting end face side without excessively restricting the effective resonator length, namely, the length of the region in which current injection into the active layer is effected. Thereby, non-radiative recombination at the laser emitting end face can be effectively restricted and at the same time the current density in the device can be controlled to be low. As a result, the COD level can be remarkably improved and a high output power semiconductor laser device can be realized.
Especially in a small-sized semiconductor laser device having a short resonator length (whole length of the device), either of the current non-injection length and the effective resonator length can be also set to be a sufficient length. Consequently, the COD level of the small-sized semiconductor laser device can be improved and a small-sized high output power semiconductor laser device can be realized.
A semiconductor laser device according to a second aspect of the present invention comprises a lower cladding layer, an active layer and an upper first cladding layer stacked in this order on a compound semiconductor substrate, a ridge-shaped upper second cladding layer provided on the upper first cladding layer, a light confining layer provided on both sides of the upper second cladding layer, and a contact layer provided on the upper second cladding layer, and the device has two end faces thereof with respect to the longitudinal direction of the ridge-shaped upper second cladding layer, namely, a laser emitting end face and a reflective end face, and two current non-injection regions each having no contact layer are provided respectively in the vicinity of both of the laser emitting end face and the reflective end face in such a manner that the current non-injection length, which is the length of the current non-injection region along the longitudinal direction of the upper second cladding layer, is longer on the laser emitting end face side than on the reflective end face side.
The effect of the COD level improvement can be obtained without necessarily completely omitting the current non-injection region on the reflective end face side. By making the current non-injection length on the laser emitting end face side longer than that on the reflective end face side, unnecessary current non-injection length on the reflective end face side is reduced. The effective resonator length can be made the longer by a length corresponding to the reduced length of the current non-injection length on the reflective end face side, so that the current density in the region of the effective resonator length can be lowered. Accordingly, in comparison with the case in a conventional structure, heat generation is more restricted and thereby the COD level can be improved. As a result, a high output power semiconductor laser device can be realized. Further, since the COD level of a small-sized semiconductor laser device can be also improved, a small-sized high output power semiconductor laser device can be realized.
It is preferable that the current non-injection length on the laser emitting end face side is equal to or more than two times the current non-injection length on the reflective end face side.
As the current non-injection length on the laser emitting end face side becomes longer with respect to the current non-injection length on the reflective end face side, the abovementioned effect of the COD level improvement becomes higher. When the current non-injection length on the laser emitting end face side is equal to or more than two times the current non-injection length on the reflective end face side, the effect of the COD level improvement becomes apparent in comparison with the COD level of a conventional structure.
The substrate 1 may be a GaAs compound semiconductor substrate and the lower cladding layer may be an Alx1Ga(1-x1)As layer.
The active layer may be a single layer of Aly1Ga(1-y1) As, a composite layer formed of Aly11Ga(1-y11)As and Aly12Ga(1-y12)As layers or a composite layer formed of Aly1Ga(1-y1)As and GaAs layers. When the active layer is a MQW (Multi Quantum Well) active layer, it is a composite layer formed of the abovementioned compositions.
The upper first cladding layer may be an Alx2Ga(1-x2)As layer, and the ridge-shaped upper second cladding layer may be an Alx3Ga(1-x3)As layer. The light confining layer may be an Aly2Ga(1-y2)As layer, and the contact layer may be a GaAs layer.
The abovementioned and other objects, features and effects of the present invention will become more apparent from the following description of the embodiments given with reference to the appended drawings.